Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
Some capacitive fingerprint sensors passively read out the capacitance between the sensing structures and the finger. This, however, requires a relatively large capacitance between sensing structure and finger. Therefore such passive capacitive sensors are typically provided with a very thin protective layer covering the sensing structures, which makes such sensors rather sensitive to scratching and/or ESD (electro-static discharge).
U.S. Pat. No. 7,864,992 discloses a capacitive fingerprint sensing system in which a driving signal is injected into the finger by pulsing a conductive structure arranged in the vicinity of the sensor array and measuring the resulting change of the charge carried by the sensing structures in the sensor array.
This type of so-called active capacitive fingerprint sensing systems generally enable measurement of the capacitance between finger and sensing structures with a much higher signal-to-noise ratio than the above-mentioned passive systems. This, in turn, allows for a considerably thicker protective coating and thus for more robust capacitive fingerprint sensors that can be included in items subjected to considerable wear, such as mobile phones.
However, there is still room for improvement. In particular, it would be desirable to provide for fingerprint sensing through even thicker protective coatings and/or for further improved performance in respect of signal-to-noise ratio.